This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Apolipoprotein E (ApoE) has three common variants among humans, differing at two residues in the 299-residue protein. Variant ApoE2 has cysteines at residues 112 and 158;ApoE3 has a cysteine at 112 and an arginine at 158;and ApoE4 has arginines at both residues. No high resolution structure of any variant has been determined, owing to the oligomeric properties of the full protein. The first X-ray crystal structure of ApoE 23-164 revealed a four helix bundle structure of the N-terminal domain. No high resolution structure of the isolated C-terminal domain has been determined, and it is thought that this domain is the primary region of oligomeric interaction. ApoE4 carries the highest risk of any genetic factor for Alzheimer's disease (AD). There is evidence for several mechanisms of ApoE4-associated AD risk, though there is no consensus. Significantly, ApoE4 has a higher preference of forming VLDL lipoproteins than does the other common ApoE variants, and this has been attributed to a conformation involving an N-terminal:C-terminal domain interaction enhanced in ApoE4 by a possible salt bridge between R61 and E255. There is a significant need for a fundamental understanding of the lipid-free ApoE structure, to help unravel the physiological consequence of mutation C158R. The central problem inhibiting the structural characterization of ApoE is oligomerization. We have access to two mutants of ApoE3, monomeric to 20 mg/mL, and possessing similar lipid-binding and LDL receptor-binding properties of ApoE3. We intend to study the oligomerization as a starting point in a long-term project to understand the interactions of ApoE and the Abeta polypeptides.